This invention relates to the field of fiber optic gyros or interferometric fiber optic gyros, IFOGS and more particularly to the field of modulation systems and methods for such gyros. IFOGS operating in a closed loop arrangement typically use a serrodyne ramp modulation system and typically do not intentionally modulate the intensity of the light source driving the Y-coupler and fiber optic coil.
Each of the embodiments presented herein uses a combination of intensity modulation of the light source driving the Sagnac interferometer and a concurrent phase modulation of the light circulating in cw (clockwise) and ccw (counterclockwise) beams in the fiber optic coil. This innovative modulation arrangement will be seen to offer an alternative to the serrodyne modulation technique. A first embodiment of the invention uses an MIOC (multifunction integrated optics chip) depicted with a Yxe2x80x94Y or 2xc3x972 coupler known in the field of multifunction integrated optics chips, such as those having integrated optic circuits formed on Lithium Niobate (LiNbO3) substrates. Multiple functions are incorporated on a single device eliminating losses and errors associated with individual interface optical coupling.
A second embodiment eliminates the higher cost MIOC and the 2xc3x972 coupler and substitutes therefore a combination using a lower cost 3xc3x973 coupler and a PZT phase modulator and two extra detectors. Both topologies rely on a combination of light source intensity and optical phase modulation at a common frequency with angular rate being determined from the phase difference angle between the intensity modulation signal and the optical phase modulation signal.
A first embodiment of the phase and intensity modulated IFOG comprises a light source that responds to an intensity modulation signal and provides an intensity modulated light signal. A Sagnac interferometer has a fiber optic coil of single mode fiber. The coil has a first and a second end, and receives and circulates a cw (clockwise) and a ccw (counter clockwise) beam. The coil has a sensitive axis that is normal to the plane of the coil. The interferometer responds to an intensity modulated light signal at a first input port, and a phase modulation signal at a phase modulation input terminal and outputs a non-reciprocal interference signal.
A PSD detector responds to the non-reciprocal interference signal and provides a demodulated bias signal. An intensity signal generator provides an intensity modulation signal at an output. A phase signal generator proves a phase modulation signal to the Sagnac interferometer phase modulation input terminal. The intensity signal generator and the phase signal generator have a common frequency. The phase difference xcex8 between the phase modulation signal and the intensity modulation signal is adjusted in response to at least a first computer control signal.
A computer has digitizing sampler coupled to receive and digitize successive sample values of the demodulated bias signal. The computer executes a program characterized to adjust the first computer control signal to adjust the phase difference xcex8 between the intensity modulation signal and the phase modulation signal to drive the amplitude of the sample values of the demodulated bias signals to a minimum. The computer program successively calculates and outputs the input rotation rate to the sensitive axis as a function of the phase angle xcex8 between the intensity modulation signal and the phase modulation signal.
In a more particular embodiment of the phase and intensity modulated IFOG has an optical coupler that receives the intensity modulated light signal at a first port. The optical coupler has a second from which it sources intensity modulated light signal and into which it receives an optical non-reciprocal interference signal. The coupler has an output port from which it sources a portion of the optical non-reciprocal interference signal.
An MIOC (multifunction integrated optics circuit) is included that has an input port coupled to the optical coupler second port, and an input wave-guide coupled to the input port that extends to a junction at which it bifurcates into at least a first and a second output wave-guide. The first and second output wave-guides are coupled to the fiber optic coil first and a second end from which they launch respective cw and ccw beams. The first and second ends receive respective ccw and cw beams after the beams transition the coil. Electrode means comprising modulator plates are coupled to the phase modulation input terminal. The electrode means responds to the phase modulation signal by phase modulating the launched and received cw and ccw beams. The returning cw and ccw beams destructively combine at the junction to form the non-reciprocal interference signal. The non-reciprocal interference signal is coupled from the MIOC input port to the optical coupler second port.
In this more particular embodiment, the PSD system further comprises a detector coupled to the optical coupler output port to receive the portion of non-reciprocal interference signal and converts the optical non-reciprocal interference signal into a buffered composite error signal. A PSD detector synchronously detects the buffered composite error signal and outputs the demodulated bias signal.
In yet an even more particular first embodiment, a transit time signal generator provides a transit time modulation signal with a frequency equal to half the reciprocal of the transit time of the coil. An adder sums the transit time modulation signal and the phase modulation signal and outputs a composite phase modulation signal to the phase modulation input terminal. The phase modulation input terminal on the interferometer receives the composite phase modulation signal and modulates the launched and received cw and ccw beams. In an even more particular embodiment, the PSD comprises a detector system coupled to receive the non-reciprocal intensity, detect it and provide a composite error signal to a preamplifier. The pre-amplifier buffered the input composite error signal and provides a buffered composite error signal to a LPF (low-pass filter). The LPF filters the input signal and provided a buffered and filtered composite error signal to the PSD (phase sensitive detector). The PSD detector system is also driven by the transit time modulation signal and synchronously demodulates the buffered and filtered composite error signal formed from the non-reciprocal interference signal and outputs the demodulated bias signal to a digitizing sampler.
In a second embodiment, of the phase and intensity modulated IFOG, the Sagnac interferometer has a 3xc3x973 coupler with first, second and third, fourth and fifth output ports, and an input port. The input port is coupled to receive the intensity-modulated light. The intensity modulated light is divided into first, second and third portions by the 3xc3x973 coupler and output from the 3xc3x973 coupler""s first, second and third output ports. The first and second output ports are coupled to the first and second ends of the fiber optic coil to launch cw and ccw beams and receive respective ccw and cw low-level non-reciprocal interference signals.
The PSD detector system that responds to the non-reciprocal interference signal in the first embodiment further comprises a first second and third detector and pre-amplifier combination. The first detector and pre-amplifier are coupled to be responsive to the ccw non-reciprocal interference signal to provide a first buffered composite error signal. The second detector and pre-amplifier combination is responsive to the cw non-reciprocal interference signal for providing a second buffered composite error signal. Each detector and pre-amplifier also provides a respective filtered composite error signal.
A modulating means responds to the phase modulation signal by modulating the launched and received cw and ccw beams. The 3xc3x973 coupler destructively combines the received ccw and cw beams to form first and second non-reciprocal interference signals and outputs a portion of the signals from the 3xc3x973 coupler""s fourth and fifth output ports. In a more particular embodiment, the modulation means further comprises a PZT fiber optic coil modulator with an input terminal coupled to be phase modulation signal for modulating the cw (clockwise) and a ccw (counter clockwise) beams.
In the more particular embodiment, the PSD detector system has a first LPF (low-pass filter) responsive to the first buffered composite error signal for providing a first buffered and filtered composite error signal. A first PSD (phase sensitive detector) responds to the first buffered and filtered composite error signal by providing a first demodulated bias signal. A second LPF responds to the second buffered composite error signal by providing a second buffered and filtered composite error signal. A second PSD responds to the second buffered and filtered composite error signal by providing a second demodulated bias signal. The digitizing sampler is coupled to receive and periodically digitize each first and second buffered and filtered composite error signal and each first and second filtered composite error signal, their respective values being coupled to the computer. The third detector and pre-amplifier are coupled to the 3xc3x973 coupler""s third output port to sample the intensity of the intensity modulated light source and provide a third filtered composite error signal. The digitizing sampler is coupled to receive and periodically digitize the third filtered composite error signal and provide a series of digitized third filtered composite error signals to the computer.
In a still more particular embodiment of the second embodiment o the IFOG, the computer program continues to repeat the routine of adjusting the angle xcex8 to minimize the value of the difference between the first and second demodulated bias signals followed by the step of calculating the angular rate being sensed by the Sagnac Interferometer. The computer first calculates the value of the variable Pbias from the equation:
Pbias=0.5*xcfx860*xcex5*cos(xcex8)Radians
where xcex80 and xcex5 are known constants. The computer then substitutes the value obtained for Pbias into the following equation to calculate the IFOG sensed rate input:       Pbias          SSF      *      3600      *      1.0125        =      sensed    ⁢          xe2x80x83        ⁢    rate    ⁢          xe2x80x83        ⁢    input    ⁢          xe2x80x83        ⁢    in    ⁢          xe2x80x83        ⁢          deg      /      sec      
where SSF is a known constant.